Pipe ‘A’ takes 15 minutes to fill a tank. Pipe ‘B’ takes 25 minutes to empty the same tank. If pipe ‘B’ is opened 11 minutes after pipe ‘A’, then find the total time taken to fill the tank in this manner.

Explanation

Let the capacity of the tank be ‘75’ litres {LCM (15, 25)} Efficiency of pipe ‘A’ = (75/15) = 5 litres/minute Efficiency of pipe ‘B’ = (75/15) = 3 litres/minute Quantity of water filled by pipe ‘A’ in 11 minutes = 11 × 5 = 55 litres Quantity of water to be filled = 75 – 55 = 20 litres Time taken by pipe ‘A’ and ‘B’ together to fill 20 litres of water = {20/(5 – 3)} = 10 minutes Total time taken = 11 + 10 = 21 minutes